This invention relates to supercapacitor devices.
Supercapacitors, also known as ultracapacitors, are energy storage devices characterized in having very high capacitance (and therefore energy density) compared with conventional capacitors, and very high power densities compared with batteries. Due to their high capacitance and high power, supercapacitors can be effective energy storage and power supply devices for a wide variety of applications. These applications include a range of consumer electronics, wireless communication devices, electric vehicles and fuel cell vehicles. Supercapacitors are especially useful in applications in which rapid energy release and/or rapid energy capture are needed. Thus, supercapacitors are used to boost acceleration and capture braking energy in electric or hybrid vehicles.
Supercapacitors can be classified as electric double layer capacitors (EDLCs), pseudocapacitors and hybrid types. The EDLCs store charge electrostatically, or “non-Faradaically”, without a transfer of charge between the electrode and electrolyte. The electrodes in EDLC types are very high surface area carbon electrodes, which can be fabricated from, for example, activated carbon, carbon nanotubes, carbon aerogels, carbon nanofibers, graphene and various composites containing one or more of those materials. The capacitance of these materials is highly dependent on their surface area.
EDLCs generally have high power densities but low energy densities. Pseudocapacitors, on the other hand, potentially have much higher energy densities than the EDLCs, at the cost of some power density. Each of these phenomena relates to the Faradaic mechanism by which pseudocapacitors store and release energy. Among the materials that have been proposed as pseudocapacitors are various transition metal oxides (such as V2O5, RuO2, MnO2, TiO2 and NiO), Ni(OH)2, other transition metal compounds such as TiS2 and BeTe3, and certain other metal oxides such as SnO2. Unlike the EDLCs, pseudocapacitors store and release energy through the transfer of charge between the electrode surface and the electrolyte. This charge transfer mechanism is slower than the physical charge storage mechanism of the EDLCs, which results in lower power densities. The pseudocapacitor materials tend to have high electrical resistivity, which is detrimental for high-power capacitance and cycling performance. In addition, high energy densities can only be obtained when the pseudocapacitor material has been fabricated with a very high surface area, which is difficult to do in practice. As a result, attempts have been made to form nanocomposites in which the pseudocapacitor material is dispersed into or coated onto a more conductive, high surface area substrate.
Hybrid supercapacitors have been proposed as a way to overcome the problem of producing high surface area pseudocapacitors, and to try to combine the high power density of an EDLC with the high energy density of a pseudocapacitor. In one approach, nanoparticles or nanocoatings of a pseudocapacitor material are applied to a high surface area carbon substrate. The carbon substrate provides capacitance via a non-Faradaic process, and also provides a conductive substrate for the pseudocapacitor particles. See, e.g., Wang et al., Dalton Trans., 2011, 40, 6388 (CeO2 nanoparticles on a graphene substrate) and Ghosh et al., Adv. Funct. Mater. 2011, 21, 2541-2547 (3-18 nm V2O5 coatings on a carbon-nanofiber paper).
Atomic layer deposition (ALD) is a thin film deposition technique that is capable of depositing conformal thin films on high-aspect-ratio substrates and nanoparticles, among other substrates. ALD processes permit close control over film thickness, and in most cases produces a film that is chemically bonded to the substrate. ALD techniques have been used to produce a TiO2-graphene composite. See Meng et al., “Controllable synthesis of graphene-based titanium dioxide nanocomposites by atomic layer deposition”, Nanotechnology 22 (2011) 165602. Meng et al. found that the structure of the applied titanium dioxide varied with the deposition temperature and with the amount of titanium dioxide that was applied (which is expressed in terms of the number of reaction cycles that were performed). The titanium dioxide at first forms 2-3 nm particles. As more reaction cycles are performed, the particles grow and eventually form a continuous film. Lower deposition temperatures (150-200° C.) tended to produce an amorphous film, whereas higher deposition temperatures (200-250° C.) produced anatase material. Meng does not describe any electrochemical testing of the composite.